1. Field of the Invention
The present invention relates to the field of anti-reflective coating for reducing reflection the surfaces of optical articles, such as displays, optical lenses, windows, optical polarizers and transparent films. More specifically, it relates to an anti-reflective coating comprising a fluoroelastomer containing a non-fluorinated cross-linking agent, where the coating has a low refractive index and good adhesion properties.
2. Description of Related Art
Optical materials are characterized by their refractive index. Whenever light travels from one material to another of different index, some of the light is reflected. Unwanted reflections can be substantially reduced by providing an anti-reflective coating on the surface of an optical article at a specified thickness. For an optical article with refractive index n, in order to reach the maximum effectiveness, the coating should have the optical thickness (the physical thickness multiplied by its own refractive index) about a quarter of the wavelength of the incoming light and have a refractive index of the square root of n. Most optical articles have refractive index ranging from 1.4 to 1.6.
It is known that the low reflective index polymer layers used in anti-reflective films can be derived from fluorine containing polymers. The refractive index of fluorinated polymer coating layers can be dependent upon the volume percentage of fluorine contained within the layer. Increased fluorine content in the layers typically decreases the refractive index of the coating layer. Thus, attention has been focused on increasing fluoropolymer content in anti-reflective coatings. See US Patent Application Publication 2005/0182199 to 3M, which discloses a fluoro-alkyl containing multi-olefinic crosslinker.
However, fluoropolymers have very poor adhesion to common substrates like plastics and glass. Various modifications have been made in order to improve their adhesion to a substrate. For instance, U.S. Patent Publication No. 2003/0068486, also assigned to 3M, discloses an anti-reflective coating which includes a layer of fluorinated material, which imparts the anti-reflective properties, covalently attached to the surface of the hardcoat layer.
Nonetheless, there still exists a need for a low refractive index anti-reflective coating which has good adhesion, as well as good scratch resistance.